Wireless power transfer system

ABSTRACT

In a wireless power transfer system, a resonant circuit is formed on the secondary coil side, phase information of a resonant current flowing in the resonant circuit is detected, and, based on this phase information, a driving frequency is determined so that the current phase of a driving current flowing in a primary coil slightly delays from the voltage phase, thereby driving the primary coil. A Q value determined based on a leakage inductance of the secondary coil, a capacitance of a resonant capacitor, and an equivalent load resistance is set to a value greater than or equal to a value determined by Q=2/k2 (k is a coupling coefficient).

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of the U.S. patent application Ser.No. 15/310,222 filed Nov. 10, 2016 (now U.S. Pat. No. 10,243,406 issuedon Mar. 26, 2019), which is the National Stage of InternationalApplication No. PCT/JP2014/004827 having an International Filing Date of19 Sep. 2014, which designated the United States of America, and whichInternational Application was published under PCT Article 21 (2) as WOPublication 2015/173850 A1, and which claims priority from and thebenefit of International Application No. PCT/JP2014/002552, filed on 14May 2014, the disclosures of which are incorporated herein by referencein their entireties.

DESCRIPTION 1. Field

The presently disclosed embodiment relates to a wireless power transfersystem and, in particular, relates to a wireless power transfer systemin which a primary coil connected to a high-frequency power source and asecondary coil connected to a load are disposed so as to be isolatedfrom each other with a coupling coefficient k, thereby supplying thepower from the primary coil to the secondary coil in a non-contactmanner.

In the specification of the parent application of the presentapplication, a term “leakage inductance” is used is some parts. However,according to JIS C 5602:1986 Glossary of Passive Components forElectronic Equipment PP. 34, No. 4305(http://kikakurui.com/c5/C5602-1986-01.html#34), it is appropriate torecite the term as “short-circuit inductance” or “leakage inductance(short-circuit inductance)”. Therefore, the term “leakage inductance(short-circuit inductance)” will be used in this specification.

2. Brief Description of Related Developments

In recent years, after the magnetic field resonance (magnetic resonance)type wireless power transfer (non-contact power feeding) was proposed,its application has been rapidly spreading. In particular, greatinterest has been shown for power transfer between a primary coil and asecondary coil in the case where a coupling coefficient k between thecoils is small and further the coupling coefficient k changessignificantly.

FIG. 24 is a block diagram showing a configuration of a conventionalmagnetic field resonance type wireless power transfer system and FIG. 25is an equivalent circuit diagram thereof.

A wireless power transfer system 200 includes a primary-side circuit(Source Electronics) 210 and a secondary-side circuit (DeviceElectronics) 230.

The primary-side circuit 210 includes an AC/DC conversion circuit 213configured to convert an alternating current supplied from analternating current (AC) power source (AC Mains) 211 into a directcurrent, a high-frequency driving circuit 215 configured to convert thisdirect current (DC) into a predetermined high frequency (RF) and toamplify and output it, a primary-side resonator (Source Resonator) 219supplied with this high frequency as driving power, and an impedancematching circuit (Impedance Matching Networks) 217 configured to performimpedance matching with the primary-side resonator 219.

The secondary-side circuit 230 includes a secondary-side resonator(Device Resonator) 231, an impedance matching circuit (IMN) 233, a RF/DCrectifying circuit (RF/DC Rectifier) 235 configured to convert a highfrequency (RF) into a direct current and to rectify and output it, and aload (Load) 237 supplied with the rectified direct current power.

The primary-side resonator 219 includes a primary coil and a primaryresonance capacitor, while the secondary-side resonator 231 includes asecondary coil and a secondary resonance capacitor.

In the magnetic field resonance type wireless power transfer, bymatching the resonance frequencies of the primary-side resonator 219 andthe secondary-side resonator 231 to resonate both resonators, highlyefficient power transfer is achieved even between the coils distancedfrom each other.

Further, in the magnetic field resonance type, the respective resonators219 and 231 are controlled by matching the impedance conditions usingthe impedance matching circuits (IMNs) 217 and 233.

In the equivalent circuit diagram shown in FIG. 25, Vg, Rg, Cs, Ls, andRs of the primary-side circuit 210 respectively denote a high-frequencydriving voltage, an equivalent resistance of the high-frequency drivingcircuit, a capacitance of the resonance capacitor, a self-inductance ofthe primary coil, and an equivalent resistance of the primary coil,while RL, Cd, Ld, and Rd of the secondary-side circuit respectivelydenote an equivalent resistance of the load, a capacitance of theresonance capacitor, a self-inductance of the secondary coil, and anequivalent resistance of the secondary coil. Further, M denotes a mutualinductance between the primary coil and the secondary coil.

Herein, in the magnetic field resonance type, it is essential to providethe primary-side resonator 219 shown in FIG. 24, which is the mostdistinctive feature of the magnetic field resonance type. Therefore, asshown in FIG. 25, the primary resonance capacitor Cs is the essentialcomponent.

In theoretical consideration of the wireless power transfer, it can besaid that a coupling coefficient k of a leakage magnetic fluxtransformer formed by a primary coil and a secondary coil changes andfurther that a load also changes simultaneously. Further, it can be saidthat since there are many variable magnetic parameters in terms ofconstituent electronic circuits, it is very difficult to simultaneouslyachieve high efficiency, stability, reliability, and so on in thewireless power transfer. In addition, under the EMC (Electro-MagneticCompatibility) regulatory arrangements in recent years, it is necessaryto employ the soft-switching ZVS (Zero Voltage Switching) techniqueconfigured for reducing noise power (noise electric field strength).

Accordingly, even if an attempt is made to obtain optimal values ofparameters of components of a driving circuit, values of the respectiveparameters do not necessarily cooperate with each other and are often ina trade-off relationship to conflict with each other, and therefore, itis extremely difficult to simultaneously achieve optimal values of therespective parameters.

Consequently, in the conventional wireless power transfer, the powercontrol is achieved by sacrificing some of the parameters. Currently, apractical value is obtained for the magnitude of power transferred inthe wireless power transfer, but there is a problem that heat generationin a driven primary coil is large. This heat generation is almost due tocopper loss and thus it is aimed to minimize this copper loss.

In the wireless power transfer, when resonance circuits are provided onboth the primary side and the secondary side, a resonance frequency ofthe primary side is constant regardless of a distance between coils,while a resonance frequency of the secondary side changes when thedistance between the coils is changed to cause a change in couplingcoefficient. This is given by the following formula, where L₂ is aninductance of a secondary coil, Cp is a resonance capacitance, and k isa coupling coefficient.

$\begin{matrix}{f_{2} = {\frac{1}{2\pi\sqrt{\left( {1 - k^{2}} \right)L_{2}C_{p}}}.}} & \left\lbrack {{Formula}\mspace{14mu} 1} \right\rbrack\end{matrix}$

Therefore, in the conventional magnetic field resonance type, resonancefrequencies of the primary side and the secondary side coincide witheach other only when the distance between the coils is a predetermineddistance, but they differ from each other when the distance between thecoils is other than the predetermined distance.

Consequently, when the primary coil and the secondary coil are in apredetermined positional relationship, practically sufficient efficiencycan be obtained, while, when the distance between the coils deviatesfrom a predetermined distance relationship or when a center axis of theprimary coil and a center axis of the secondary coil are offset fromeach other, the power factor as seen from the primary coil decreasesrapidly. In this case, there is a problem that while the power transferis enabled, its efficiency is poor so that heat generation furtherincreases.

Further, there is also a problem that, in order to ensure ZVS operationof a switching element configured to drive the primary coil, the phaserelationship between the primary coil and the secondary coil should bein a specific range and thus that the ZVS operation can only be achievedunder very limited conditions.

Therefore, in the conventional wireless power transfer, in order toconstantly ensure the ZVS operation, measures are taken, such aschanging a driving frequency according to a certain program whilemonitoring a condition of a load.

SUMMARY

It is an object of the presently disclosed embodiment to solve part ofthe above-mentioned various problems caused by the conventional magneticfield resonance type.

In particular, it is an object of the presently disclosed embodiment toprovide a wireless power transfer system that can simultaneouslymaintain power factors as seen from both the driving means side of aprimary coil and the primary coil in an excellent relationship toachieve efficient power transfer even when the condition is changed,such as when the distance between the coils is changed or when a centeraxis of the primary coil and a center axis of the secondary coil areoffset from each other.

Further, it is an object of the presently disclosed embodiment toprovide a wireless power transfer system that performs driving with highrobustness and high efficiency by reducing both a copper loss and aswitching loss with a simple circuit by automatically obtaining adriving frequency optimal for performing efficient power transfer.

According to a first aspect of the presently disclosed embodiment, thereis provided a wireless power transfer system in which a primary coilconnected to a high-frequency power source and a secondary coilconnected to a load are disposed so as to be isolated from each otherwith a coupling coefficient k, thereby supplying power from the primarycoil to the secondary coil in a non-contact manner, the systemcomprising: a resonance current phase detection means forming aresonance circuit by connecting a resonance capacitor to the secondarycoil and configured to detect phase information of a resonance currentflowing through the resonance capacitor; a phase information transfermeans configured to transfer the detected phase information withoutphase delay; and a driving circuit configured to determine, based on thephase information, a driving frequency so that a current phase of adriving current flowing in the primary coil slightly delays from avoltage phase of a driving voltage applied to the primary coil, therebydriving the primary coil, wherein a Q value determined based on aleakage inductance of the secondary coil, a capacitance of the resonancecapacitor, and an equivalent load resistance on the secondary coil sideis set to a value greater than or equal to a value determined by Q=2/k².

According to a second aspect of the presently disclosed embodiment,there is provided a wireless power transfer system in which a primarycoil connected to a high-frequency power source and a secondary coilconnected to a load are disposed so as to be isolated from each otherwith a coupling coefficient k, thereby supplying power from the primarycoil to the secondary coil in a non-contact manner, the systemcomprising: a resonance current phase detection means forming aresonance circuit by connecting a resonance capacitor to the secondarycoil and configured to detect phase information of a resonance currentflowing in the secondary coil; a phase information transfer meansconfigured to transfer the detected phase information without phasedelay; and a driving circuit configured to determine, based on the phaseinformation, a driving frequency so that a current phase of a drivingcurrent flowing in the primary coil slightly delays from a voltage phaseof a driving voltage applied to the primary coil, thereby driving theprimary coil, wherein a Q value determined based on a leakage inductanceof the secondary coil, a capacitance of the resonance capacitor, and anequivalent load resistance on the secondary coil side is set to a valuegreater than or equal to a value determined by Q=2/k².

According to a third aspect of the presently disclosed embodiment, thereis provided a wireless power transfer system in which a primary coilconnected to a high-frequency power source and a secondary coilconnected to a load are disposed so as to be isolated from each otherwith a coupling coefficient k, thereby supplying power from the primarycoil to the secondary coil in a non-contact manner, the systemcomprising: a resonance current phase detection means forming aresonance circuit by connecting a resonance capacitor to the secondarycoil and configured to detect, from the primary coil, phase informationof a resonance current flowing in the resonance circuit; a phaseinformation transfer means configured to transfer the detected phaseinformation without phase delay; and a driving circuit configured todetermine, based on the phase information, a driving frequency so that acurrent phase of a driving current flowing in the primary coil slightlydelays from a voltage phase of a driving voltage applied to the primarycoil, thereby driving the primary coil, wherein a Q value determinedbased on a leakage inductance of the secondary coil, a capacitance ofthe resonance capacitor, and an equivalent load resistance on thesecondary coil side is set to a value greater than or equal to a valuedetermined by Q=2/k².

According to a fourth aspect of the presently disclosed embodiment,there is provided a wireless power transfer system in which a primarycoil connected to a high-frequency power source and a secondary coilconnected to a load are disposed so as to be isolated from each otherwith a coupling coefficient k, thereby supplying power from the primarycoil to the secondary coil in a non-contact manner, the systemcomprising: a resonance current phase detection means forming aresonance circuit by connecting a resonance capacitor to the secondarycoil and configured to detect phase information of a resonance currentbased on a waveform obtained by superimposing and combining one of awaveform of a resonance current flowing through the resonance capacitor,a waveform of a resonance current flowing in the secondary coil, and awaveform of a resonance current flowing in the primary coil, and aninverted integrated waveform of the one of the waveforms; a phaseinformation transfer means configured to transfer the detected phaseinformation without phase delay; and a driving circuit configured todetermine, based on the phase information, a driving frequency so that acurrent phase of a driving current flowing in the primary coil slightlydelays from a voltage phase of a driving voltage applied to the primarycoil, thereby driving the primary coil, wherein a Q value determinedbased on a leakage inductance of the secondary coil, a capacitance ofthe resonance capacitor, and an equivalent load resistance on thesecondary coil side is set to a value greater than or equal to a valuedetermined by Q=2/k².

According to a fifth aspect of the presently disclosed embodiment, inany of the first to fourth aspects, a filter configured to removedistortion included in a waveform of the resonance current and toextract only a fundamental wave is provided.

According to a sixth aspect of the presently disclosed embodiment, inany of the first to fifth aspects, the driving circuit includes aswitching means configured to drive the primary coil and the switchingmeans performs power control by changing an on-off duty cycle thereof,turning on the switching means based on the phase information, andturning off the switching means after a lapse of a predetermined time.

According to a seventh aspect of the presently disclosed embodiment, inthe first or fourth aspect, the resonance current phase detection meansdetects the phase information from a current flowing through asmall-capacity capacitor connected in parallel to the resonancecapacitor.

According to an eighth aspect of the presently disclosed embodiment,there is provided a wireless power transfer system including a primarycoil connected to a high-frequency power source, a secondary coilconnected to a load, and a third coil disposed close to the secondarycoil or including the secondary coil as an autotransformer and wound soas to step down a voltage induced in the secondary coil, wherein theprimary coil and the secondary coil are disposed so as to be isolatedfrom each other with a coupling coefficient k, thereby supplying powerfrom the primary coil to the third coil via the secondary coil in anon-contact manner, the system comprising: a resonance current phasedetection means forming a resonance circuit by connecting a resonancecapacitor to the secondary coil and configured to detect phaseinformation of a resonance current flowing through the resonancecapacitor; a phase information transfer means configured to transfer thedetected phase information without phase delay; and a driving circuitconfigured to determine, based on the phase information, a drivingfrequency so that a current phase of a driving current flowing in theprimary coil slightly delays from a voltage phase of a driving voltageapplied to the primary coil, thereby driving the primary coil, wherein aQ value determined based on a leakage inductance of the secondary coil,a capacitance of the resonance capacitor, and an equivalent loadresistance on the secondary coil side is set to a value greater than orequal to a value determined by Q=2/k².

According to a ninth aspect of the presently disclosed embodiment, thereis provided a wireless power transfer system including a primary coilconnected to a high-frequency power source, a secondary coil connectedto a load, and a third coil disposed close to the secondary coil orincluded in an autotransformer including the secondary coil, the thirdcoil being wound so as to step down a voltage induced in the secondarycoil, wherein the primary coil and the secondary coil are disposed so asto be isolated from each other with a coupling coefficient k, therebysupplying power from the primary coil to the secondary coil in anon-contact manner, the system comprising: a resonance current phasedetection means forming a resonance circuit by connecting a resonancecapacitor to the secondary coil and configured to detect phaseinformation of a resonance current flowing in the secondary coil; aphase information transfer means configured to transfer the detectedphase information without phase delay; and a driving circuit configuredto determine, based on the phase information, a driving frequency sothat a current phase of a driving current flowing in the primary coilslightly delays from a voltage phase of a driving voltage applied to theprimary coil, thereby driving the primary coil, wherein a Q valuedetermined based on a leakage inductance of the secondary coil, acapacitance of the resonance capacitor, and an equivalent loadresistance on the secondary coil side is set to a value greater than orequal to a value determined by Q=2/k².

According to a tenth aspect of the presently disclosed embodiment, thereis provided a wireless power transfer system including a primary coilconnected to a high-frequency power source, a secondary coil connectedto a load, and a third coil disposed close to the secondary coil orincluded in an autotransformer including the secondary coil, the thirdcoil being wound so as to step down a voltage induced in the secondarycoil, wherein the primary coil and the secondary coil are disposed so asto be isolated from each other with a coupling coefficient k, therebysupplying power from the primary coil to the secondary coil in anon-contact manner, the system comprising: a resonance current phasedetection means forming a resonance circuit by connecting a resonancecapacitor to the secondary coil and configured to detect, from theprimary coil, phase information of a resonance current flowing in theresonance circuit; a phase information transfer means configured totransfer the detected phase information without phase delay; and adriving circuit configured to determine, based on the phase information,a driving frequency so that a current phase of a driving current flowingin the primary coil slightly delays from a voltage phase of a drivingvoltage applied to the primary coil, thereby driving the primary coil,wherein a Q value determined based on a leakage inductance of thesecondary coil, a capacitance of the resonance capacitor, and anequivalent load resistance on the secondary coil side is set to a valuegreater than or equal to a value determined by Q=2/k².

According to an eleventh aspect of the presently disclosed embodiment,there is provided a wireless power transfer system including a primarycoil connected to a high-frequency power source, a secondary coilconnected to a load, and a third coil disposed close to the secondarycoil or included in an autotransformer including the secondary coil, thethird coil being wound so as to step down a voltage induced in thesecondary coil, wherein the primary coil and the secondary coil aredisposed so as to be isolated from each other with a couplingcoefficient k, thereby supplying power from the primary coil to thesecondary coil in a non-contact manner, the system comprising: aresonance current phase detection means forming a resonance circuit byconnecting a resonance capacitor to the secondary coil and configured todetect phase information of a resonance current based on a waveformobtained by superimposing and combining one of a waveform of a resonancecurrent flowing through the resonance capacitor, a waveform of aresonance current flowing in the secondary coil, and a waveform of aresonance current flowing in the primary coil, and an invertedintegrated waveform of the one of the waveforms; a phase informationtransfer means configured to transfer the detected phase informationwithout phase delay; and a driving circuit configured to determine,based on the phase information, a driving frequency so that a currentphase of a driving current flowing in the primary coil slightly delaysfrom a voltage phase of a driving voltage applied to the primary coil,thereby driving the primary coil, wherein a Q value determined based ona leakage inductance of the secondary coil, a capacitance of theresonance capacitor, and an equivalent load resistance on the secondarycoil side is set to a value greater than or equal to a value determinedby Q=2/k².

According to a twelfth aspect of the presently disclosed embodiment, inany of the eighth to eleventh aspects, a filter configured to removedistortion included in a waveform of the resonance current and toextract only a fundamental wave is provided.

According to a thirteenth aspect of the presently disclosed embodiment,in any of the eighth to twelfth aspects, the driving circuit includes aswitching means configured to drive the primary coil and the switchingmeans performs power control by changing an on-off duty cycle thereof,turning on the switching means based on the phase information, andturning off the switching means after a lapse of a predetermined time.

According to a fourteenth aspect of the presently disclosed embodiment,in the eighth or eleventh aspect, the resonance current phase detectionmeans detects the phase information from a current flowing through asmall-capacity capacitor connected in parallel to the resonancecapacitor.

According to the presently disclosed embodiment, a better result can beobtained by not providing a resonance circuit is not provided to aprimary coil. If a resonance circuit is provided only to a secondarycoil by connecting a resonance capacitor to the secondary coil, aresonance current does not flow in the primary coil so that heatgeneration in the primary coil is suppressed. However, it is optional toutilize a primary side resonance by, for example, using technologiessuch as a variable capacitor in combination.

Further, being not bound by a resonance frequency of the primary coil,it is possible to automatically select, as a driving frequency, afrequency with the best power factor as seen from the primary coil sideso that the robustness is largely improved.

Further, in the presently disclosed embodiment, since ZVS operation isconstantly maintained, it is possible to employ a half-bridge circuitconfiguration as a driving circuit so that a system can be achieved witha simple circuit configuration compared to the conventional wirelesspower transfer system.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram showing a configuration of a main part of awireless power transfer system according to the presently disclosedembodiment.

FIG. 2 is a circuit diagram showing examples of a switching means foruse in the presently disclosed embodiment.

FIG. 3 is a diagram showing a configuration of a detecting portionincluding a resonance current phase detection means according to thepresently disclosed embodiment.

FIG. 4 is a diagram showing another example of the resonance currentphase detection means according to the presently disclosed embodiment.

FIG. 5 is a diagram showing still another example of the resonancecurrent phase detection means according to the presently disclosedembodiment.

FIG. 6 is a diagram for explaining that phase information becomesinaccurate due to distortion included in a resonance current phasewaveform.

FIG. 7 is a conceptual diagram showing that harmonic distortion isremoved from a resonance current phase waveform.

FIG. 8 is a conceptual diagram showing changes of various waveforms whena phase delay has occurred in resonance current phase information.

FIG. 9 is a diagram showing waveforms of currents flowing in theswitching means where ZVS operation is performed in the case where aphase delay of resonance current phase information is small.

FIG. 10 is a diagram showing waveforms of currents flowing in theswitching means where ZVS operation is not performed in the case where aphase delay of resonance current phase information is large.

FIG. 11 is a diagram showing one example of a circuit configured tocorrect the phase of resonance current phase information.

[FIGS. 12A, B] are diagrams for explaining the correction of the phaseinformation.

FIG. 13 is a specific circuit diagram for performing the phasecorrection.

FIG. 14 is another specific circuit diagram for performing the phasecorrection.

FIG. 15 is still another specific circuit diagram for performing thephase correction.

FIG. 16 is a diagram for explaining the relationship between the currentphase of a resonance capacitor and the current phase of a primary coil.

FIG. 17 is a diagram in which minimum Q values required for obtaining apower factor of 1 are derived.

FIG. 18 is a diagram for explaining that a required minimum Q valuegradually approaches k²Q=2 as k decreases.

FIG. 19 is a diagram showing that when Q=2/k², a current phase curve ofa primary coil crosses 0 degrees.

FIG. 20 is a diagram for explaining one example of the power controlaccording to the presently disclosed embodiment.

FIG. 21 is a diagram for explaining another example of the power controlaccording to the presently disclosed embodiment.

FIG. 22 is a diagram for explaining the relationship with the currentphase of a primary coil when the power control of the presentlydisclosed embodiment is performed.

FIG. 23 is a diagram showing one example of a circuit that can improvethe efficiency while maintaining the interchangeability with the Qistandard.

FIG. 24 is a block diagram showing a configuration of a conventionalmagnetic field resonance type wireless power transfer system.

FIG. 25 is an equivalent circuit diagram of the conventional magneticfield resonance type wireless power transfer system.

DETAILED DESCRIPTION

Hereinbelow, an aspect of a wireless power transfer system according tothe presently disclosed embodiment will be described in detail withreference to the accompanying drawings.

FIG. 1 is a block diagram showing a configuration of a main part of oneaspect of a wireless power transfer system 100 according to thepresently disclosed embodiment.

The primary side includes a primary coil (Primary coil) 110 and adriving circuit 120 connected to the primary coil 110 via a capacitorCc. The driving circuit 120 includes a driving means (Driving means) 122and a switching means (Switching means) 124. The switching means 124 isformed as a bridge circuit including transistor elements Q1 to Q4.

The secondary side includes a secondary coil (Secondary coil) 140disposed so as to be isolated from the primary coil 110 with a couplingcoefficient k, a resonance capacitor (Cp) 150 connected to the secondarycoil 140 to form a resonance circuit, and a resonance current phasedetection means (Resonance current phase detection means) 160 configuredto detect phase information of a resonance current flowing through theresonance capacitor (Cp) 150. In the presently disclosed embodiment, aphase information transfer means 170 configured to transfer the phaseinformation detected by the resonance current phase detection means 160to the driving circuit 120 without phase delay is provided and thedriving circuit 120 determines a driving frequency based on this phaseinformation and drives the primary coil 110. The phase informationtransfer means 170 includes a phase information transmission means(Phase information Transmitter) 172 and a phase information receptionmeans (Phase information Receiver) 174.

The secondary side is connected to a load not shown.

In the presently disclosed embodiment, only the resonance circuit thatis formed by connecting the resonance capacitor (Cp) 150 to thesecondary coil 140 is essential, and sufficiently efficient powertransmission can be achieved without providing a series resonancecapacitor to the primary coil 110.

The capacitor Cc shown in FIG. 1 is provided for the purpose of merelyblocking a direct current, but is not provided as a resonance capacitor.

More specifically, primary-side resonance may be provided as a means forimproving the presently disclosed embodiment, which, however, isoptional but not essential in the presently disclosed embodiment.

If the driving timing of the switching means 124 can be preciselycontrolled so that the balance of currents that flow through therespective transistor elements Q1 to Q4 can be maintained, the capacitorCc may be omitted.

In the presently disclosed embodiment, phase information of a resonancecurrent flowing in the resonance circuit formed by the secondary coil140 and the resonance capacitor (Cp) 150 connected in parallel or inseries to the secondary coil 140 is detected by the resonance currentphase detection means 160, then the phase information is sent from thephase information transmission means 172 on the secondary side to thephase information reception means 174 on the primary side, then thedriving timing of the driving means 122 is determined based on the phaseinformation.

In the presently disclosed embodiment, since no resonance circuit isprovided on the primary side, the number of windings of the primary coil110 differs compared to the conventional magnetic field resonance typethat requires a primary-side resonance circuit.

While the phase information transfer means 170 transfers phaseinformation detected by the resonance current phase detection means 160“without phase delay”, a phase delay inevitably exists in any detectionmeans or transfer means and, therefore, “without phase delay” hereinmeans to minimize a phase delay as much as possible and it issatisfactory if the primary-side driving means and the secondary sidecan share an absolute time.

Next, the respective components shown in the block diagram of FIG. 1will be described.

Based on phase information transferred from the phase informationtransfer means 170, the driving means 122 determines a driving frequencyso that the current phase of a driving current flowing in the primarycoil 110 slightly delays from the voltage phase of a driving voltageapplied to the primary coil 110, thereby driving the switching means124. Detailed operation thereof will be described later.

The switching means 124 is formed by switching elements (Q1 to Q4) suchas two or four transistors and drives the primary coil 110 via the DCblocking capacitor Cc.

FIG. 2 is a circuit diagram showing examples of the switching means 124,wherein A shows a full-bridge circuit and B shows a half-bridge circuit.In the presently disclosed embodiment, the switching means 124 may beformed by the full-bridge circuit or the half-bridge circuit.

In the conventional wireless power transfer, the phase relationshipbetween a driving voltage applied to the primary coil 110 and a drivingcurrent flowing in the primary coil 110, as seen from the switchingmeans 124, largely changes depending on a driving frequency, andtherefore, it has been difficult to maintain ZVS under variousconditions and thus to prevent hard switching. Accordingly, afull-bridge circuit has been preferably employed for the reason thatabnormal voltage is difficult to occur even at the occurrence of hardswitching.

In the presently disclosed embodiment, a half-bridge circuit, which isconventionally said to be difficult to employ due to the occurrence ofhard switching, can be safely employed. This is because, in thepresently disclosed embodiment, it is possible to maintain ZVS undervarious conditions. Details will be described later. A full-bridgecircuit is advantageous only in its high power utilization efficiencyand is not an essential condition in the presently disclosed embodiment.Hereinbelow, a description will be given of the case where thefull-bridge circuit is used as the switching means 124.

FIG. 3 is a diagram showing a configuration of a detecting portionincluding the resonance current phase detection means 160 according tothe presently disclosed embodiment.

The resonance capacitor (Cp) 150 and a small-capacity capacitor (Cps)155 are connected in parallel to the secondary coil 140.

The resonance current phase detection means 160 may be configured todirectly detect phase information of a current flowing through theresonance capacitor (Cp) 150 as shown in FIG. 1 or may be configured(160 a) to detect phase information of a current flowing through theparallelly connected small-capacity capacitor (Cps) 155 as shown in FIG.3. This is because since a voltage applied to the resonance capacitor(Cp) 150 and a voltage applied to the small-capacity capacitor (Cps) 155connected in parallel thereto are the same, phase information ofcurrents flowing through the respective capacitors are the same. In manycases, the current that flows through the resonance capacitor (Cp) 150is very large and, therefore, if this current is directly detected, theresonance current phase detection means 160 should be formed bylarge-capacity parts, while, if the current flowing through thesmall-capacity capacitor (Cps) is detected, the resonance current phasedetection means 160 can be formed by small-capacity parts.

The phase information of the resonance current detected by the resonancecurrent phase detection means 160 is transferred to the primary-sidedriving circuit 120 by the resonance current phase transfer means 170.

As a configuration of the resonance current phase transfer means 170,various configurations can be considered. It may be optical couplingusing a LED and a phototransistor, phase information may be transferredby modulating a magnetic circuit by a digitized signal of the phaseinformation, or it may be a wireless means using a high-frequencycarrier electromagnetic wave. If the phototransistor is used as a lightreceiving portion, a delay due to stored electric charge is large and,therefore, a means that uses the phototransistor by saturating it is notpreferable. Accordingly, the unsaturated operation is preferable for thephototransistor and the constant voltage operation for suppressing themirror effect is further preferable. More preferably, a pin photodiodemay be used and further it may be used by applying a reverse biasthereto, thereby achieving high-speed operation.

As shown in FIG. 4, the resonance current phase detection means 160 maybe configured (160 b) to detect a current flowing in the secondary coil140. In this case, however, a phase component of a current flowingthrough a load is also included and combined in detected phaseinformation. Since the phase component of the current flowing throughthe load delays by 90 degrees from a resonance current phase, thesevectors are combined so that some phase delay occurs in the phaseinformation. When the primary coil 110 is driven based on this phaseinformation, ZVS operation is not achieved so that hard switching tendsto occur.

When the hard switching occurs, high-frequency parasitic oscillationoccurs in the elements (Q1 to Q4) of the switching means 124. In thiscase, EMI (Electro-Magnetic Interference), particularly noisepower/noise electric field strength (radiation), increases. Therefore, acountermeasure is required, which will be described later.

As shown in FIG. 5, the resonance current phase detection means 160 maybe configured (160 c) to detect a current flowing in the primary coil110. A shows a type in which power is extracted to a load R from thesecondary coil 140, B shows a type in which the secondary coil includesa third coil 190 as an autotransformer and power is extracted to a loadR from the third coil 190, and C shows a type in which power isextracted to a load R from a third coil 190 provided as an independentcoil close to the secondary coil 140. The types B and C in which thethird coil 190 is provided on the secondary side will be described indetail later. In these cases, hard switching often occurs so that phasecorrection is separately required, which will be described later.

There is a case where distortion is generated in a detected resonancecurrent to cause a delay of resonance current phase information. Suchdistortion makes the resonance current phase information lack accuracyso that the driving timing of the primary coil 110 becomes inaccurateand that the peak of the resonance frequency cannot be correctlydetected.

FIG. 6 shows an example in which a third harmonic wave is included in aresonance current, and is an explanatory diagram for explaining thatphase information becomes inaccurate due to this distortion. In such acase, by extracting only a fundamental component by an appropriatefilter means and setting it as resonance current phase information, itis possible to enhance the accuracy of the phase information.Alternatively, it is possible to use only phase information of afundamental wave by digitally series-expanding the resonance current.

FIG. 7 is a conceptual diagram showing that harmonic distortion isremoved from a resonance current phase waveform by providing theresonance current phase detection means 160 with a filter (Fundamentalwaveform filter) 165 configured to extract only a fundamental wave.

Next, a description will be given of setting a Q value of thesecondary-side resonance circuit, which is important in the presentlydisclosed embodiment.

The wireless power transfer largely differs from power feeding by ageneral leakage transformer. In the case of the power feeding by theleakage transformer, a coupling coefficient (k) between a primary coiland a secondary coil is almost constant under all driving conditions,while a coupling coefficient (k) largely changes in the wireless powertransfer. In the conventional power feeding by the leakage transformer,a Q value of a secondary-side resonance circuit does not need to be setso high.

On the other hand, in the case of the wireless power transfer, since thecoupling coefficient (k) changes, a high Q value is necessary when thecoupling coefficient (k) is small.

When the Q value is low, it is difficult to keep the power factor of theprimary coil good in the case where the conditions in use are largelychanged. On the other hand, when the Q value is too high, likewise inthe case where the conditions in use are largely changed, it isdifficult to satisfy the condition that a driving frequency isdetermined so that the current phase of a driving current flowing in theprimary coil slightly delays from the voltage phase of a driving voltageapplied to the primary coil, thereby driving the primary coil (i.e. theZVS operation condition).

In the presently disclosed embodiment, a high Q value is required forincreasing the robustness. However, as the Q value becomes higher, thefull width half maximum becomes much narrower so that even a slightfrequency error causes a problem. Therefore, a phase delay (or a timedelay) in the resonance current phase information transfer means shouldbe minimized as much as possible. When a delay in the resonance currentphase information transfer means has occurred, the following occurs.

FIG. 8 is a conceptual diagram showing changes of various waveforms whena phase delay has occurred in resonance current phase information.

a is a waveform of a resonance current of the resonance circuit, b is awaveform of phase information detected by the resonance current phasedetection means 160, c is a waveform of phase information output fromthe phase information reception means 174, and d is a waveform of acurrent flowing in the primary coil 110. The waveform c output from thephase information reception means 174 is delayed from the waveform b andbecomes a driving waveform for the switching means 124 almost as it is.

FIG. 9 is a diagram showing waveforms of currents flowing in theswitching means 124 where ZVS operation is performed in the case where aphase delay of resonance current phase information is small.

When no delay or an extremely small delay exists in a waveform of thephase information reception means 174 (c), since the timing of a currentflowing in the primary coil 110 is slightly advanced compared to theswitching timing of the switching means 124 (d), current waveforms ofthe switching elements Q1 and Q2 (e, f) of the switching means 124achieve ZVS operation. In this case, the center tap voltage of theswitching means 124 becomes a complete square wave (g).

On the other hand, FIG. 10 is a diagram showing waveforms of currentsflowing in the switching means 124 where ZVS operation is not performedin the case where a phase delay of resonance current phase informationis large.

Since the timing of a current flowing in the primary coil 110 is delayedcompared to the switching timing of the switching means 124 (d), currentwaveforms of the switching elements Q1 and Q2 (e, f) do not achieve ZVSoperation so that a unique pulse waveform due to rebound is generated inthe center tap voltage of the switching means 124 (g). The generation ofthis rebound waveform may break the switching elements Q1 and Q2 and thedriving means 122 and may cause the occurrence of EMI.

From the above, in the presently disclosed embodiment, a phase delay inthe phase information transfer means 170 should be minimized as much aspossible. When a phase delay cannot be avoided, the following phasecorrection means is used.

FIG. 11 is a diagram showing one example of a circuit configured tocorrect the phase of resonance current phase information.

A resonance current waveform detection circuit 162, an integrationinverting circuit 164 configured to invert and integrate or to integrateand invert a waveform of a detected resonance current, and an addingcircuit 166 configured to superimpose and combine an output from theresonance current waveform detection circuit 162 and an output from theintegration inverting circuit 164 are provided in the resonance currentphase detection means 160. The phase of an inverted integrated waveformis advanced by 90 degrees compared to that of the waveform of theoriginal resonance current. Accordingly, by detecting resonance currentphase information based on a waveform obtained by superimposing andcombining the waveform of the original resonance current and itsinverted integrated waveform in a proper ratio, it is possible to obtainresonance current phase information corrected in a phase advancingdirection. Then, this is sent to the phase information reception means174 via the phase information transmission means 172.

FIGS. 12A and 12B are diagrams for explaining the phase informationcorrection described above.

a is original resonance current waveform information and b is itsinverted integrated waveform. c is a combined waveform obtained bysuperimposing and combining a and b. Phase information d corrected bythe combined waveform in a phase advancing direction is a waveform thattransfers the original resonance current waveform information a withoutdelay.

The integration inverting circuit 164 may be formed by using anoperational amplifier or may be configured to perform inversion by usinga transformer and then to perform integration by using a capacitor (C)and a resistor (R).

FIG. 13 is a diagram showing a specific circuit diagram of the resonancecurrent phase detection means 160 a for performing the phase correction.

The phase of resonance current waveform information a is advanced bybeing inverted and integrated by an integration inverting circuit 164via a buffer amplifier and then being combined by a combining circuit166, thereby obtaining a corrected waveform c (see FIG. 12A).

FIG. 14 is a diagram showing another specific circuit diagram of theresonance current phase detection means 160 b, wherein, paying attentionto the fact that the voltage at both ends of the resonance capacitor Cp150 has an integrated waveform with respect to a resonance currentwaveform, this voltage is properly divided and inverted, therebyperforming phase correction.

The phase of resonance current waveform information a is advanced bybeing combined with the inverted resonance capacitor voltage by acombining circuit 166, thereby obtaining a corrected waveform c (seeFIG. 12B).

FIG. 15 is a diagram showing still another specific circuit diagram ofthe resonance current phase detection means 160 c. In this example, aresonance current waveform detected from the primary coil 110 via acurrent transformer 167 and its inverted integrated waveform arecombined, thereby performing phase correction. A resonance current phasewaveform detected on the primary side is a and an inverted integratedresonance current phase waveform is b, wherein a corrected waveform c isobtained by combining them.

It is also possible to use a differentiated, rather than integrated,waveform without inversion, which is included in the meaning ofinversion and integration as a proper design matter in the presentlydisclosed embodiment. However, in many cases, harmonic components areemphasized and superimposed on a differentiated waveform and, therefore,it can be said that it is preferable compared to the case where anintegrated waveform is used. The phase correction means may properlycombine the respective resonance current phase waveforms a and theirinverted integrated waveforms b of 160 a, 160 b, and 160 c, or may besuch that the respective circuits of the current detection means 160 aand 160 b are replaced with each other.

The description has been given above of the case where resonance currentwaveform information and resonance current phase information areprocessed in an analog manner. However, the phase transfer means in thepresently disclosed embodiment aims to achieve that the primary-sidedriving means and the secondary side share an absolute time, and thephase correction means in the presently disclosed embodiment issatisfactory if a phase-advanced corrected waveform is resultantlyobtained based on a waveform including resonance current phaseinformation.

If an absolute time can be shared by some means, it is possible toobtain corrected phase information based on a difference from theabsolute time and thus to obtain a corrected phase waveform. That is, itis needless to say that digital processing is enabled based on theknowledge described above.

Next, a description will be given of setting a Q value of the resonancecircuit formed on the secondary side, which is important in thepresently disclosed embodiment.

As described earlier, in the case of the wireless power transfer, sincethe coupling coefficient (k) changes, a particularly large Q value isrequired when the coupling coefficient (k) is small. When the Q value islow, in the case where the conditions in use are largely changed, it isdifficult to drive the primary coil while satisfying the condition thatthe current phase of a driving current flowing in the primary coilslightly delays from the voltage phase of a driving voltage applied tothe primary coil.

In order to solve this, it is not preferable to fix a driving frequency(so-called fixed-frequency type or called a separate excitation type),while it is essential to control the driving circuit based on phaseinformation of a resonance current flowing through the resonancecapacitor of the secondary-side resonance circuit, a resonance currentflowing in the secondary coil, or a resonance current reflected on theprimary coil. As a result, there is no alternative but to make thedriving frequency variable. It is described in JP2002-272134 that thedriving frequency is changed depending on the resistance component of aload in the magnetic field resonance type wireless power transfer.

In the invention described in JP2002-272134, by detecting a loadresistance, an optimal driving frequency is obtained by pre-programmedprediction information or calculation and a saturable inductor, therebydriving a driving means. However, according to this method, while thepower factor as seen from a switching means of a driving circuitconfigured to drive a primary coil approaches 1, the power factor asseen from the primary coil does not approach 1, and therefore, whileheat generation of the switching means is suppressed, the power factoras seen from the primary coil side is very poor and causes heatgeneration of the primary coil.

In the presently disclosed embodiment, in order to derive a conditionfor eliminating a resonance circuit on the primary coil side and causingthe power factor as seen from the primary coil to approach 1, a Q valueof the resonance circuit on the secondary coil side is set higher thannormal.

FIG. 16 is a diagram for explaining the relationship between the currentphase of the resonance capacitor connected to the secondary coil and thecurrent phase of the primary coil, wherein Q values required when thecoupling coefficient (k) is changed are obtained by simulation.

a shows the current phase of the primary coil, wherein the ordinate axisrepresents the phase angle and the abscissa axis represents the drivingfrequency. b shows the current phase of the secondary-side resonancecapacitor, wherein the ordinate axis represents the phase angle and theabscissa axis represents the driving frequency. c shows the transferratio, wherein the ordinate axis represents the transfer ratio and theabscissa axis represents the driving frequency. A transfer ratio, whenmultiplied by a ratio between the number of windings of the primary coiland the number of windings of the secondary coil, generally shows astep-up ratio.

It is seen that when the coupling coefficient (k) is 0.5, the conditionthat the phase of a current flowing in the primary coil slightly delaysfrom the phase of a driving voltage of the driving circuit configured todrive the primary coil is satisfied by setting the Q value to 8 or more.In the example of FIG. 16, when the coupling coefficient (k) is 0.5, afrequency at which the transfer ratio becomes maximum is 85 kHz. At thisfrequency, a delay angle Δθ of a current flowing in the primary coil is25 degrees or less so that cos θ, i.e. the power factor, becomes 0.9 ormore. Therefore, it can be said that 85 kHz is the optimal drivingfrequency.

At this optimal driving frequency, the phase of a resonance currentflowing through the secondary-side resonance capacitor is 0 degrees.That is, if this resonance current phase information is transferred tothe driving circuit via the phase information transfer means withoutphase delay to drive the driving circuit, the switching means can bedriven automatically at the optimal driving frequency. Further, sincethis switching condition is also ZVS operation, even if the switchingmeans has a half-bridge configuration, stable ZVS operation can beachieved.

As described above, according to the presently disclosed embodiment, thedriving circuit is driven at a driving frequency determined based onphase information of a resonance current and automatically performs ZVSoperation. However, since a high Q value is required in the wirelesspower transfer, if there is even a slight phase delay (or time delay),ZVS operation is not achieved so that phase correction is required.

Next, a description will be given of setting a target Q value.

A Q value of the resonance circuit is determined as follows, based on aleakage inductance (short-circuit inductance) (L) of the secondary coil,a capacitance (C) of the resonance capacitor, and an equivalent loadresistance (R) on the secondary coil side.

$\begin{matrix}{{Q = {{\frac{1}{R}\sqrt{\frac{L}{C}}} = {\frac{\omega\; L}{R} = {\frac{1}{\omega\;{CR}}\mspace{14mu}\left( {{series}\mspace{14mu}{resonance}\mspace{14mu}{circuit}} \right)}}}}{Q = {{R\sqrt{\frac{C}{L}}} = {\frac{R}{\omega\; L} = {\omega\;{CR}\mspace{14mu}\left( {{parallel}\mspace{14mu}{resonance}\mspace{14mu}{circuit}} \right)}}}}} & \left\lbrack {{Formula}\mspace{14mu} 2} \right\rbrack\end{matrix}$

In the presently disclosed embodiment, while either series connection orparallel connection may be used for connection between the secondarycoil and the resonance capacitor, a description will be given below ofan example of parallel connection. This is because the resonanceproviding the resonance capacitor in parallel to the secondary coil,which is one of the presently disclosed embodiment, becomes seriesresonance when seen from the driving side and parallel resonance whenseen from the load side, which thus is a modified resonance circuit.This is called by various names such as Serial Parallel-LoadedResonance. Parallel resonance is applied to a calculation of Q in thiscase. That is, in order to set the Q value to a predetermined highvalue, the capacitance C of the resonance capacitor is set large and theleakage inductance (short-circuit inductance) L of the secondary coil isset small compared to the equivalent load resistance R.

The definition of a leakage inductance (short-circuit inductance) L_(sc)in the presently disclosed embodiment is determined by the followingformula.L _(sc) =L ₂·(1−k ²)  [Formula 3]

where L₂ is an inductance of the secondary coil.

The leakage inductance is variously defined by industrial societies oracademic societies in various countries and is not standardized, suchas, for example,L _(g) =L ₂·(1−k)  [Formula 4]

where L_(e) is a leakage inductance.

In the presently disclosed embodiment, the resonance circuit formed onthe secondary side by connecting the secondary coil and the resonancecapacitor includes one of a configuration in which the resonancecapacitor is connected in parallel to the secondary coil and aconfiguration in which the resonance capacitor is connected in series tothe secondary coil. A configuration for extracting the power to a loadfrom a third coil will be described later.

In the presently disclosed embodiment, a target Q value of thesecondary-side resonance circuit is set to a value greater than or equalto a value satisfying Q=2/k² where k is a coupling coefficient.

FIG. 17 is a diagram in which minimum Q values required for obtainingthe power factor as seen from the primary coil are derived bysimulation. The abscissa axis represents the driving frequency. On theordinate axis, a shows the current phase of the primary coil withrespect to the switching voltage of the primary coil, b shows theresonance current phase of the secondary-side resonance capacitor, and cshows the transfer ratio (transfer coefficient).

FIG. 18 is a diagram for explaining that the minimum Q value requiredfor obtaining a power factor of 1 as seen from the primary coilgradually approaches a relationship of k²Q=2 as the coupling coefficientk decreases.

As is clear from FIG. 18, the value of k²·Q gradually approaches 2 as kdecreases, but never exceeds 2. Accordingly, in the wireless powertransfer, when the distance between the primary coil and the secondarycoil is large so that the coupling coefficient k is sufficiently small,the Q value of the secondary-side resonance circuit for obtaining apower factor of 1 as seen from the primary coil satisfiesk ² ·Q=2  [Formula 5]

Herein, in consideration of the operation of detecting, from the primarycoil, phase information of a resonance current flowing in thesecondary-side resonance circuit, a current phase curve of the primarycoil should cross an abscissa axis of 0 degrees and, in this case, thecurrent phase curve should cross it so that the phase changes from plus,i.e. capacitive, to minus, i.e. inductive, as the driving frequencyincreases.

Accordingly, when the minimum Q value is set to a Q value exceeding[Equation 5], a current phase curve of the primary coil surely crossesan abscissa axis of 0 degrees. This is explained in FIG. 19.

In FIG. 19, a, b, and c respectively correspond to those in FIG. 17. InFIG. 19, when coupling coefficient k=0.7, a current phase curve of theprimary coil, as shown at a, crosses an abscissa axis of 0 degrees sothat the phase changes from plus, i.e. capacitive, to minus, i.e.inductive, as the driving frequency increases.

While k=0.7 in the above description, the same description also appliesto the case where k takes other values. Herein, when the power factor isset to just 1, hard switching occurs even when the current phase of theprimary coil is slightly advanced. Therefore, the normal operating pointis set to a point where the current phase of the primary coil isslightly inductive, i.e. slightly delayed. This is known as a ZVSoperation condition.

In this case, the power factor is sufficiently satisfactory if the delayangle of the phase is in a range from 0 degrees to −30 degrees andtherefore it can be said that the efficiency is quite excellent. Whenphase information of a resonance current flowing through the resonancecapacitor is used, if there is no phase delay or time delay in thisphase information, by driving the primary-side switching means inaccordance with this phase information, even when the couplingcoefficient is small, the system automatically operates at a ZVSoperation point in the state where the Q value is high.

Next, the power control in the presently disclosed embodiment will bedescribed.

In general, the power control is performed by changing the duty cycle ofthe switching means so as to be smaller than 50%.

FIG. 20 is a diagram for explaining one example of the power controlaccording to the presently disclosed embodiment. a shows phaseinformation of a resonance current, b shows a current waveform of theprimary coil, and c and d show gate control voltages when the switchingmeans (Q2, Q1) are FETs, IGBTs, or the like.

In the presently disclosed embodiment, the power control is performedby, based on phase information of a resonance current, turning on theswitching means of the driving circuit configured to drive the primarycoil and turning off the switching means after the lapse of apredetermined time. In this case, hard switching is generally concernedabout. However, in the presently disclosed embodiment, since the currentphase of the primary coil is already delayed from the switching phase(see b), the current phase when the duty control is performed is furtherdelayed from the on phase (see c and d) so that hard switching does notoccur. In this event, some dead time (Dead time) is required forpreventing (Q2 and Q1) from turning on simultaneously, which, however,is generally performed so that a description thereof is omitted.

FIG. 21 is a diagram for explaining another example of the power controlaccording to the presently disclosed embodiment. In this power control,the duty control is performed for only one of the switching means oronly a pair of the switching means which turn on simultaneously, and theother or the other pair is controlled by an inverted signal. In thisevent, some dead time (Dead time) is likewise required for the other orthe other pair controlled by the inverted signal (see d).

Such a control method is called unbalanced half-bridge control orunbalanced full-bridge control. In this control method, even harmonicvoltage tends to be generated in the secondary coil. However, in thepresently disclosed embodiment, since the Q value is set very high, thevoltage of the secondary coil is approximately a sine wave and thusthere is no problem.

FIG. 22 is a diagram for explaining the relationship with the currentphase of the primary coil when the power control of the presentlydisclosed embodiment is performed.

In the control method of the presently disclosed embodiment, the drivingfrequency when controlled increases and, as is also clear from FIG. 22,it is seen that as the frequency increases, the on phase of a drivingvoltage is further delayed from the phase of phase information of aresonance current. While the power factor decreases, since the transferratio decreases simultaneously, the controllable power range is greatlywidened.

In the wireless power transfer system described above, the resonancecircuit is formed by connecting the resonance capacitor to the secondarycoil. On the other hand, for example, in the Qi standard using theelectromagnetic induction type, the secondary-side resonance circuit asin the presently disclosed embodiment is not essential. Therefore, inorder to apply the presently disclosed embodiment while ensuring theinterchangeability with the Qi standard, a description will be given ofthe case where, like in FIG. 5C, a third coil is provided side by sidewith the secondary coil and the power is extracted to a load from thisthird coil.

FIG. 23 is a diagram showing one example of a circuit that can improvethe efficiency while maintaining the interchangeability with the Qistandard. Like in FIG. 5C, a third coil 190 may be provided as anindependent coil close to a secondary coil 140, or alternatively, asshown in FIG. 23 or FIG. 5B, a third coil 190 may be included as anautotransformer in a secondary coil 140 and may be wound so as to stepdown a voltage induced in the secondary coil 140.

A resonance capacitor (Cp) 150 is connected to the secondary coil 140and, in the case of a Qi interchangeable standard, a switch 181 isturned off to disable resonance operation, while, when improving theefficiency, the switch 181 is turned on so that Cp operates as theresonance capacitor 150. Simultaneously, a switch 183 on the secondarycoil 140 side is turned off, while a switch 185 is turned on. A load notshown is connected between the switches 183 and 185 and secondary-sideGND. When the switch 181 is turned on, the Q value of the resonancecircuit increases.

With this operation, since a frequency with an excellent power factoroccurs in a primary coil 110, a resonance current waveform detected by aresonance current phase detection means 160 c provided on the primaryside and its inverted integrated waveform are combined to perform phasecorrection and, based on its resonance current phase information, adriving circuit of a high-frequency power source is driven. In thecircuit of FIG. 23, the switches 181, 183, and 185 respectively usesFETs or transistors via diodes. However, being not limited thereto, anyelements can be used if they are capable of switching operation.

When the third coil is wound so as to step down a voltage induced in thesecondary coil as in FIG. 5C, FIG. 5B, or FIG. 23, the load resistanceconnected to the third coil is impedance-converted in inverse proportionto the square of a step-down ratio so that a high-value equivalent loadresistance is virtually connected to the secondary coil. Therefore, theQ value of the resonance circuit can be set high in accordance with thisratio. By setting this step-down ratio, the Q value can be easily set toa desired value greater than or equal to a value determined by Q=2/k².

Further, since the resonance current is inversely proportional to thenumber of windings, the copper loss due to the resonance currentdecreases in proportion to the square of the current and therefore it ispossible to reduce heat generation so that the efficiency is improved.

DESCRIPTION OF REFERENCE NUMERALS

-   -   100 WIRELESS POWER TRANSFER SYSTEM    -   110 PRIMARY COIL    -   120 DRIVING CIRCUIT    -   122 DRIVING MEANS    -   124 SWITCHING MEANS    -   140 SECONDARY COIL    -   150 RESONANCE CAPACITOR    -   155 SMALL-CAPACITY CAPACITOR    -   160 RESONANCE CURRENT PHASE DETECTION MEANS    -   165 FILTER    -   170 PHASE INFORMATION TRANSFER MEANS    -   190 THIRD COIL

What is claimed is:
 1. A wireless power transfer system in which aprimary coil connected to a high-frequency power source and a secondarycoil connected to a load are disposed so as to be isolated from eachother with a coupling coefficient k, thereby supplying power from theprimary coil to the secondary coil in a non-contact manner, the systemcomprising: a resonance current phase detection means forming aresonance circuit by connecting a resonance capacitor to the secondarycoil and configured to detect phase information of a resonance currentflowing through the resonance capacitor and correct the detected phaseinformation so that a phase of the detected phase information isadvanced; a phase information transfer means configured to transfer thecorrected phase information; and a driving circuit configured todetermine, based on the transferred phase information, a drivingfrequency so that a current phase of a driving current flowing in theprimary coil slightly delays from a voltage phase of a driving voltageapplied to the primary coil, thereby driving the primary coil, wherein aQ value determined based on a leakage inductance of the secondary coil,a capacitance of the resonance capacitor, and an equivalent loadresistance on the secondary coil side is set to a value greater than orequal to a value determined by Q=2/k².
 2. The wireless power transfersystem according to claim 1, comprising a filter configured to removedistortion included in a waveform of the resonance current and toextract only a fundamental wave.
 3. The wireless power transfer systemaccording to claim 1, wherein the driving circuit includes a switchingmeans configured to drive the primary coil, and wherein the switchingmeans performs power control by changing an on-off duty cycle thereof,turning on the switching means based on the phase information, andturning off the switching means after a lapse of a predetermined time.4. The wireless power transfer system according to claim 1, wherein theresonance current phase detection means detects the phase informationfrom a current flowing through a small-capacity capacitor connected inparallel to the resonance capacitor.
 5. A wireless power transfer systemin which a primary coil connected to a high-frequency power source and asecondary coil connected to a load are disposed so as to be isolatedfrom each other with a coupling coefficient k, thereby supplying powerfrom the primary coil to the secondary coil in a non-contact manner, thesystem comprising: a resonance current phase detection means forming aresonance circuit by connecting a resonance capacitor to the secondarycoil and configured to detect phase information of a resonance currentflowing in the secondary coil and correct the detected phase informationso that a phase of the detected phase information is advanced; a phaseinformation transfer means configured to transfer the corrected phaseinformation; and a driving circuit configured to determine, based on thetransferred phase information, a driving frequency so that a currentphase of a driving current flowing in the primary coil slightly delaysfrom a voltage phase of a driving voltage applied to the primary coil,thereby driving the primary coil, wherein a Q value determined based ona leakage inductance of the secondary coil, a capacitance of theresonance capacitor, and an equivalent load resistance on the secondarycoil side is set to a value greater than or equal to a value determinedby Q=2/k².
 6. A wireless power transfer system in which a primary coilconnected to a high-frequency power source and a secondary coilconnected to a load are disposed so as to be isolated from each otherwith a coupling coefficient k, thereby supplying power from the primarycoil to the secondary coil in a non-contact manner, the systemcomprising: a resonance current phase detection means forming aresonance circuit by connecting a resonance capacitor to the secondarycoil and configured to detect, from the primary coil, phase informationof a resonance current flowing in the resonance circuit and correct thedetected phase information so that a phase of the detected phaseinformation is advanced; a phase information transfer means configuredto transfer the corrected phase information; and a driving circuitconfigured to determine, based on the transferred phase information, adriving frequency so that a current phase of a driving current flowingin the primary coil slightly delays from a voltage phase of a drivingvoltage applied to the primary coil, thereby driving the primary coil,wherein a Q value determined based on a leakage inductance of thesecondary coil, a capacitance of the resonance capacitor, and anequivalent load resistance on the secondary coil side is set to a valuegreater than or equal to a value determined by Q=2/k².
 7. A wirelesspower transfer system in which a primary coil connected to ahigh-frequency power source and a secondary coil connected to a load aredisposed so as to be isolated from each other with a couplingcoefficient k, thereby supplying power from the primary coil to thesecondary coil in a non-contact manner, the system comprising: aresonance current phase detection means forming a resonance circuit byconnecting a resonance capacitor to the secondary coil and configured todetect phase information of a resonance current based on a waveformobtained by superimposing and combining one of a waveform of a resonancecurrent flowing through the resonance capacitor, a waveform of aresonance current flowing in the secondary coil, and a waveform of aresonance current flowing in the primary coil, and an invertedintegrated waveform of the one of the waveforms, and correct thedetected phase information so that a phase of the detected phaseinformation is advanced; a phase information transfer means configuredto transfer the corrected phase information; and a driving circuitconfigured to determine, based on the transferred phase information, adriving frequency so that a current phase of a driving current flowingin the primary coil slightly delays from a voltage phase of a drivingvoltage applied to the primary coil, thereby driving the primary coil,wherein a Q value determined based on a leakage inductance of thesecondary coil, a capacitance of the resonance capacitor, and anequivalent load resistance on the secondary coil side is set to a valuegreater than or equal to a value determined by Q=2/k².
 8. A wirelesspower transfer system including a primary coil connected to ahigh-frequency power source, a secondary coil connected to a load, and athird coil disposed close to the secondary coil or including thesecondary coil as an autotransformer and wound so as to step down avoltage induced in the secondary coil, wherein the primary coil and thesecondary coil are disposed so as to be isolated from each other with acoupling coefficient k, thereby supplying power from the primary coil tothe third coil via the secondary coil in a non-contact manner, thesystem comprising: a resonance current phase detection means forming aresonance circuit by connecting a resonance capacitor to the secondarycoil and configured to detect phase information of a resonance currentflowing through the resonance capacitor and correct the detected phaseinformation so that a phase of the detected phase information isadvanced; a phase information transfer means configured to transfer thecorrected phase information; and a driving circuit configured todetermine, based on the transferred phase information, a drivingfrequency so that a current phase of a driving current flowing in theprimary coil slightly delays from a voltage phase of a driving voltageapplied to the primary coil, thereby driving the primary coil, wherein aQ value determined based on a leakage inductance of the secondary coil,a capacitance of the resonance capacitor, and an equivalent loadresistance on the secondary coil side is set to a value greater than orequal to a value determined by Q=2/k².
 9. The wireless power transfersystem according to claim 8, comprising a filter configured to removedistortion included in a waveform of the resonance current and toextract only a fundamental wave.
 10. The wireless power transfer systemaccording to claim 8, wherein the driving circuit includes a switchingmeans configured to drive the primary coil, and wherein the switchingmeans performs power control by changing an on-off duty cycle thereof,turning on the switching means based on the phase information, andturning off the switching means after a lapse of a predetermined time.11. The wireless power transfer system according to claim 8, wherein theresonance current phase detection means detects the phase informationfrom a current flowing through a small-capacity capacitor connected inparallel to the resonance capacitor.
 12. A wireless power transfersystem including a primary coil connected to a high-frequency powersource, a secondary coil connected to a load, and a third coil disposedclose to the secondary coil or included in an autotransformer includingthe secondary coil, the third coil being wound so as to step down avoltage induced in the secondary coil, wherein the primary coil and thesecondary coil are disposed so as to be isolated from each other with acoupling coefficient k, thereby supplying power from the primary coil tothe secondary coil in a non-contact manner, the system comprising: aresonance current phase detection means forming a resonance circuit byconnecting a resonance capacitor to the secondary coil and configured todetect phase information of a resonance current flowing in the secondarycoil and correct the detected phase information so that a phase of thedetected phase information is advanced; a phase information transfermeans configured to transfer the corrected phase information; and adriving circuit configured to determine, based on the transferred phaseinformation, a driving frequency so that a current phase of a drivingcurrent flowing in the primary coil slightly delays from a voltage phaseof a driving voltage applied to the primary coil, thereby driving theprimary coil, wherein a Q value determined based on a leakage inductanceof the secondary coil, a capacitance of the resonance capacitor, and anequivalent load resistance on the secondary coil side is set to a valuegreater than or equal to a value determined by Q=2/k².
 13. A wirelesspower transfer system including a primary coil connected to ahigh-frequency power source, a secondary coil connected to a load, and athird coil disposed close to the secondary coil or included in anautotransformer including the secondary coil, the third coil being woundso as to step down a voltage induced in the secondary coil, wherein theprimary coil and the secondary coil are disposed so as to be isolatedfrom each other with a coupling coefficient k, thereby supplying powerfrom the primary coil to the secondary coil in a non-contact manner, thesystem comprising: a resonance current phase detection means forming aresonance circuit by connecting a resonance capacitor to the secondarycoil and configured to detect, from the primary coil, phase informationof a resonance current flowing in the resonance circuit and correct thedetected phase information so that a phase of the detected phaseinformation is advanced; a phase information transfer means configuredto transfer the corrected phase information; and a driving circuitconfigured to determine, based on the transferred phase information, adriving frequency so that a current phase of a driving current flowingin the primary coil slightly delays from a voltage phase of a drivingvoltage applied to the primary coil, thereby driving the primary coil,wherein a Q value determined based on a leakage inductance of thesecondary coil, a capacitance of the resonance capacitor, and anequivalent load resistance on the secondary coil side is set to a valuegreater than or equal to a value determined by Q=2/k².
 14. A wirelesspower transfer system including a primary coil connected to ahigh-frequency power source, a secondary coil connected to a load, and athird coil disposed close to the secondary coil or included in anautotransformer including the secondary coil, the third coil being woundso as to step down a voltage induced in the secondary coil, wherein theprimary coil and the secondary coil are disposed so as to be isolatedfrom each other with a coupling coefficient k, thereby supplying powerfrom the primary coil to the secondary coil in a non-contact manner, thesystem comprising: a resonance current phase detection means forming aresonance circuit by connecting a resonance capacitor to the secondarycoil and configured to detect phase information of a resonance currentbased on a waveform obtained by superimposing and combining one of awaveform of a resonance current flowing through the resonance capacitor,a waveform of a resonance current flowing in the secondary coil, and awaveform of a resonance current flowing in the primary coil, and aninverted integrated waveform of the one of the waveforms, and correctthe detected phase information so that a phase of the detected phaseinformation is advanced; a phase information transfer means configuredto transfer the corrected phase information; and a driving circuitconfigured to determine, based on the transferred phase information, adriving frequency so that a current phase of a driving current flowingin the primary coil slightly delays from a voltage phase of a drivingvoltage applied to the primary coil, thereby driving the primary coil,wherein a Q value determined based on a leakage inductance of thesecondary coil, a capacitance of the resonance capacitor, and anequivalent load resistance on the secondary coil side is set to a valuegreater than or equal to a value determined by Q=2/k².
 15. A wirelesspower transfer system in which a primary coil connected to ahigh-frequency power source and a secondary coil connected to a load aredisposed so as to be isolated from each other with a couplingcoefficient k, thereby supplying power from the primary coil to thesecondary coil in a non-contact manner, the system comprising: aresonance current phase detection means forming a resonance circuit byconnecting a resonance capacitor to the secondary coil, configured todetect phase information of a resonance current flowing through theresonance capacitor, and including a phase correction means configuredto, when a phase delay has occurred in the detected phase information,correct the phase delay by correcting the detected phase information ina phase advancing direction, the resonance current phase detection meansbeing configured to detect the corrected phase information as the phaseinformation of the resonance current; and a driving circuit configuredto determine, based on the corrected phase information, a drivingfrequency so that a current phase of a driving current flowing in theprimary coil slightly delays from a voltage phase of a driving voltageapplied to the primary coil, thereby driving the primary coil, wherein aQ value determined based on a leakage inductance of the secondary coil,a capacitance of the resonance capacitor, and an equivalent loadresistance on the secondary coil side is set to a value greater than orequal to a value determined by Q=2/k².
 16. The wireless power transfersystem according to claim 15, comprising a filter configured to removedistortion included in a waveform of the resonance current and toextract only a fundamental wave.
 17. The wireless power transfer systemaccording to claim 15, wherein the driving circuit includes a switchingmeans configured to drive the primary coil, and wherein the switchingmeans performs power control by changing an on-off duty cycle thereof,turning on the switching means based on the phase information, andturning off the switching means after a lapse of a predetermined time.18. The wireless power transfer system according to claim 15, whereinthe resonance current phase detection means detects the phaseinformation from a current flowing through a small-capacity capacitorconnected in parallel to the resonance capacitor.
 19. A wireless powertransfer system in which a primary coil connected to a high-frequencypower source and a secondary coil connected to a load are disposed so asto be isolated from each other with a coupling coefficient k, therebysupplying power from the primary coil to the secondary coil in anon-contact manner, the system comprising: a resonance current phasedetection means forming a resonance circuit by connecting a resonancecapacitor to the secondary coil, configured to detect phase informationof a resonance current flowing in the secondary coil, and including aphase correction means configured to, when a phase delay has occurred inthe detected phase information, correct the phase delay by correctingthe detected phase information in a phase advancing direction, theresonance current phase detection means being configured to detect thecorrected phase information as the phase information of the resonancecurrent; and a driving circuit configured to determine, based on thecorrected phase information, a driving frequency so that a current phaseof a driving current flowing in the primary coil slightly delays from avoltage phase of a driving voltage applied to the primary coil, therebydriving the primary coil, wherein a Q value determined based on aleakage inductance of the secondary coil, a capacitance of the resonancecapacitor, and an equivalent load resistance on the secondary coil sideis set to a value greater than or equal to a value determined by Q=2/k².20. A wireless power transfer system in which a primary coil connectedto a high-frequency power source and a secondary coil connected to aload are disposed so as to be isolated from each other with a couplingcoefficient k, thereby supplying power from the primary coil to thesecondary coil in a non-contact manner, the system comprising: aresonance current phase detection means forming a resonance circuit byconnecting a resonance capacitor to the secondary coil, configured todetect, from the primary coil, phase information of a resonance currentflowing in the resonance circuit, and including a phase correction meansconfigured to, when a phase delay has occurred in the detected phaseinformation, correct the phase delay by correcting the detected phaseinformation in a phase advancing direction, the resonance current phasedetection means being configured to detect the corrected phaseinformation as the phase information of the resonance current; and adriving circuit configured to determine, based on the corrected phaseinformation, a driving frequency so that a current phase of a drivingcurrent flowing in the primary coil slightly delays from a voltage phaseof a driving voltage applied to the primary coil, thereby driving theprimary coil, wherein a Q value determined based on a leakage inductanceof the secondary coil, a capacitance of the resonance capacitor, and anequivalent load resistance on the secondary coil side is set to a valuegreater than or equal to a value determined by Q=2/k².
 21. A wirelesspower transfer system including a primary coil connected to ahigh-frequency power source, a secondary coil connected to a load, and athird coil disposed close to the secondary coil or including thesecondary coil as an autotransformer and wound so as to step down avoltage induced in the secondary coil, wherein the primary coil and thesecondary coil are disposed so as to be isolated from each other with acoupling coefficient k, thereby supplying power from the primary coil tothe third coil via the secondary coil in a non-contact manner, thesystem comprising: a resonance current phase detection means forming aresonance circuit by connecting a resonance capacitor to the secondarycoil, configured to detect phase information of a resonance currentflowing through the resonance capacitor, and including a phasecorrection means configured to, when a phase delay has occurred in thedetected phase information, correct the phase delay by correcting thedetected phase information in a phase advancing direction, the resonancecurrent phase detection means being configured to detect the correctedphase information as the phase information of the resonance current; anda driving circuit configured to determine, based on the corrected phaseinformation, a driving frequency so that a current phase of a drivingcurrent flowing in the primary coil slightly delays from a voltage phaseof a driving voltage applied to the primary coil, thereby driving theprimary coil, wherein a Q value determined based on a leakage inductanceof the secondary coil, a capacitance of the resonance capacitor, and anequivalent load resistance on the secondary coil side is set to a valuegreater than or equal to a value determined by Q=2/k².
 22. The wirelesspower transfer system according to claim 21, comprising a filterconfigured to remove distortion included in a waveform of the resonancecurrent and to extract only a fundamental wave.
 23. The wireless powertransfer system according to claim 21, wherein the driving circuitincludes a switching means configured to drive the primary coil, andwherein the switching means performs power control by changing an on-offduty cycle thereof, turning on the switching means based on the phaseinformation, and turning off the switching means after a lapse of apredetermined time.
 24. The wireless power transfer system according toclaim 21, wherein the resonance current phase detection means detectsthe phase information from a current flowing through a small-capacitycapacitor connected in parallel to the resonance capacitor.
 25. Awireless power transfer system including a primary coil connected to ahigh-frequency power source, a secondary coil connected to a load, and athird coil disposed close to the secondary coil or included in anautotransformer including the secondary coil, the third coil being woundso as to step down a voltage induced in the secondary coil, wherein theprimary coil and the secondary coil are disposed so as to be isolatedfrom each other with a coupling coefficient k, thereby supplying powerfrom the primary coil to the secondary coil in a non-contact manner, thesystem comprising: a resonance current phase detection means forming aresonance circuit by connecting a resonance capacitor to the secondarycoil, configured to detect phase information of a resonance currentflowing in the secondary coil, and including a phase correction meansconfigured to, when a phase delay has occurred in the detected phaseinformation, correct the phase delay by correcting the detected phaseinformation in a phase advancing direction, the resonance current phasedetection means being configured to detect the corrected phaseinformation as the phase information of the resonance current; and adriving circuit configured to determine, based on the corrected phaseinformation, a driving frequency so that a current phase of a drivingcurrent flowing in the primary coil slightly delays from a voltage phaseof a driving voltage applied to the primary coil, thereby driving theprimary coil, wherein a Q value determined based on a leakage inductanceof the secondary coil, a capacitance of the resonance capacitor, and anequivalent load resistance on the secondary coil side is set to a valuegreater than or equal to a value determined by Q=2/k².
 26. A wirelesspower transfer system including a primary coil connected to ahigh-frequency power source, a secondary coil connected to a load, and athird coil disposed close to the secondary coil or included in anautotransformer including the secondary coil, the third coil being woundso as to step down a voltage induced in the secondary coil, wherein theprimary coil and the secondary coil are disposed so as to be isolatedfrom each other with a coupling coefficient k, thereby supplying powerfrom the primary coil to the secondary coil in a non-contact manner, thesystem comprising: a resonance current phase detection means forming aresonance circuit by connecting a resonance capacitor to the secondarycoil, configured to detect, from the primary coil, phase information ofa resonance current flowing in the resonance circuit, and including aphase correction means configured to, when a phase delay has occurred inthe detected phase information, correct the phase delay by correctingthe detected phase information in a phase advancing direction, theresonance current phase detection means being configured to detect thecorrected phase information as the phase information of the resonancecurrent; and a driving circuit configured to determine, based on thecorrected phase information, a driving frequency so that a current phaseof a driving current flowing in the primary coil slightly delays from avoltage phase of a driving voltage applied to the primary coil, therebydriving the primary coil, wherein a Q value determined based on aleakage inductance of the secondary coil, a capacitance of the resonancecapacitor, and an equivalent load resistance on the secondary coil sideis set to a value greater than or equal to a value determined by Q=2/k².